Fair Value Of Financial Instruments (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	$ 1,267,449	$ 1,143,684
Available-for-sale securities	3,070,447	3,773,508
Investments	67,982	111,379
Cash equivalents measured at NAV	19,719	77,441
Derivative contracts, Assets	7,683	7,995
Total Assets	4,433,280	5,114,007
Financial instruments sold, but not yet purchased, at fair value	947,306	778,863
Sovereign Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	14,063	32,470
Financial instruments sold, but not yet purchased, at fair value	11,429	20,236
U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	72,532	147,223
Available-for-sale securities	5,215	4,983
Financial instruments sold, but not yet purchased, at fair value	32,755	10,348
Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	25,905	76,752
Available-for-sale securities	1,764,060	2,106,938
Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	310,457	325,471
Available-for-sale securities	931,604	1,211,442
Financial instruments sold, but not yet purchased, at fair value	208,725	180,755
Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	57,911	46,802
Investments	33,046	49,978
Financial instruments sold, but not yet purchased, at fair value	36,117	38,510
State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	154,622	169,699
Available-for-sale securities	68,226	70,559
Financial instruments sold, but not yet purchased, at fair value	7
Investments In Funds Measured At NAV [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	16,559	24,549
Auction Rate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	16,632	34,789
Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	704	846
Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,041	1,217
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	42,121	13,466
Financial instruments sold, but not yet purchased, at fair value	534,817	442,402
Mortgage Backed Securities [Member] | Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	564,111	302,445
Available-for-sale securities	230,408	305,530
Financial instruments sold, but not yet purchased, at fair value	123,456	86,612
Mortgage Backed Securities [Member] | Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	25,727	29,356
Available-for-sale securities	1,219	1,568
Mortgage Backed Securities [Member] | Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	69,715	72,488
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	100,346	60,239
Available-for-sale securities	417	516
Investments	31,670	49,978
Total Assets	132,433	110,733
Financial instruments sold, but not yet purchased, at fair value	571,504	480,472
Level 1 [Member] | U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	417	516
Level 1 [Member] | Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	1,100	362
Financial instruments sold, but not yet purchased, at fair value	1,289
Level 1 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	57,125	46,411
Investments	31,670	49,978
Financial instruments sold, but not yet purchased, at fair value	35,398	38,070
Level 1 [Member] | U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	42,121	13,466
Financial instruments sold, but not yet purchased, at fair value	534,817	442,402
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	1,166,927	1,082,592
Available-for-sale securities	3,070,030	3,772,992
Investments	1,560	360
Derivative contracts, Assets	7,683	7,995
Total Assets	4,246,200	4,863,939
Financial instruments sold, but not yet purchased, at fair value	375,802	298,391
Level 2 [Member] | Sovereign Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	14,063	32,470
Financial instruments sold, but not yet purchased, at fair value	11,429	20,236
Level 2 [Member] | U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	72,532	147,223
Available-for-sale securities	4,798	4,467
Financial instruments sold, but not yet purchased, at fair value	32,755	10,348
Level 2 [Member] | Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	25,730	76,395
Available-for-sale securities	1,764,060	2,106,938
Level 2 [Member] | Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	309,357	324,867
Available-for-sale securities	931,604	1,211,442
Financial instruments sold, but not yet purchased, at fair value	207,436	180,755
Level 2 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	786	138
Investments	1,376
Financial instruments sold, but not yet purchased, at fair value	719	440
Level 2 [Member] | State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	154,622	169,699
Available-for-sale securities	68,226	70,559
Financial instruments sold, but not yet purchased, at fair value	7
Level 2 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	184	360
Level 2 [Member] | Mortgage Backed Securities [Member] | Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	564,111	302,445
Available-for-sale securities	230,408	305,530
Financial instruments sold, but not yet purchased, at fair value	123,456	86,612
Level 2 [Member] | Mortgage Backed Securities [Member] | Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	25,726	29,355
Available-for-sale securities	1,219	1,568
Level 2 [Member] | Mortgage Backed Securities [Member] | Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	69,715	72,488
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	176	853
Investments	18,193	36,492
Total Assets	18,369	37,345
Level 3 [Member] | Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	175	357
Level 3 [Member] | Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		242
Level 3 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		253
Level 3 [Member] | Auction Rate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	16,632	34,789
Level 3 [Member] | Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	704	846
Level 3 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	857	857
Level 3 [Member] | Mortgage Backed Securities [Member] | Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	$ 1	$ 1